The first violent game I ever had was Goldeneye when I was 9. Since then I have been playing a number of games like Counter-Strike/Unreal Tournament etc. I still play Battlefield 2 to date.
I remember the first film/movie that scared the shit out of me was Alien which I watched when I was.. 8 or 9. I remember coming down stairs at around 12am because the sound was really high on the TV and my mum fast asleep watching Alien. Since then from the ages of 12- now my parents have often given me 15/18's DVD/Videos for my birthday and Xmas.
I guess my point is that a large majority of my life has been exposed to video/film violence and I can honestly say that I am not a violent individual. I was never in any "serious" fights at school, just the old pushing from time to time :P, so really I have no commited any "violent" acts, apart from shooting my "friend" 5x in the head when we were paintballing
Overal; I feel that in no way that violence on the TV/Computer affects children and therefore games/films should not really be cencored to an extent e.g. Manhunt. Unless the kid is psychotic or summin